The present invention relates to superconducting filter arrangements, particularly notch filters or band reject filters which comprise a superconducting dielectric resonator and a waveguide arrangement such as e.g. a microstrip line.
One of the applications of notch filters or band reject filters is within communications systems. A particular application of such relates to multichannel microwave communications systems which operate in high frequency bands in which the size of the components is highly important.
The invention also relates to a method for filtering signals incoming to a receiving arrangement in a multichannel communication system.
Since for example frequency multiplexers, band reject filters etc. are among the key elements in multichannel communication systems, efforts have been made to find a way to reduce the insertion losses and the size of these components. For multichannel microwave communication systems operating in the 1-3 GHz frequency band insertion losses are high for presently used devices.
It is known to use YIG (Yttrium Iron Garnet) notch filters in the front end of microwave receivers to blank out intermittent interfering signals. However, the insertion losses are high. Moreover the size of such filters is large. In "High-Temperature Superconducting Microwave Devices", by Shen, Artech House, 1994 the use of high temperature superconductors is discussed for providing new possibilities to reduce the size and to improve the performance of microwave components, for example filters.
European Patent Application EP-A-0 567 407 discloses superconducting notch filters with a fixed frequency wherein half wavelength, high temperature superconducting microstrip resonators are parallel coupled to the main high temperature superconducting microstrip line. The substrates of the resonators have dielectric constants of about 10-25 at frequencies between 1-3 GHz. The length of the filters is then about 2-6 cm; the filters are thus very large and they are also expensive.
In some communication systems tuneable (switchable) notch filters are required instead of fixed frequency notch filters e.g. in order to increase the flexibility of the system. U.S. Pat. No. 4,834,498; and shows a simple dielectric resonator. The resonator is passive and it is not itself tunable. To provide tunability additional tuning means are required such as a diode. In other words, a separate biasing circuit is required. This considerably adds to the size of the arrangement. Furthermore, the device as such gets complex and the performance is not sufficiently high. In WO 93/00720 a superconducting notch filter with a microstrip resonator which is not tunable itself is illustrated. In this case optical means are used to provide tuning, which use semiconductor crystals in superconducting microstrip ring resonators coupled to the main superconducting microstrip. However, the dimensions of these arrangements are large and moreover the frequency tuning range is much too small. Both of the above mentioned documents show passive resonators and devices requiring a special bias network and additional tuning means which are coupled to a main microstrip line in the same way. The resonators cannot be in mechanical or electrical contact with the main microstrip line. If there is no coupling, there is no filter.
To summarize, both these devices need additional tuning means with a separate biasing circuit. That makes the designs large as well as complex. Furthermore, the electrical performance of the filter is negatively affected therethrough and it is also as such not as high as would be desired. For example frequencies of about 1-3 GHz the devices as disclosed in these documents would be much too large and they cannot for example be used for telecommunication purposes.
It has also been found that microwave devices can be made smaller if high dielectric constant non-linear dielectric materials such as for example Strontium Titanate (STO) are plated with superconductors such as e.g. Y--Ba--Cu--O (YBCO). WO 94/13028 discloses the use of thin single crystalline dielectric films in combination with high temperature superconductors which as such however produce too high microwave losses and moreover such devices cannot be made small enough.